1. Field of the Invention
The present invention relates to a digital signal transmission system having frame synchronization operation. The system according to the present invention is used, for example, for PCM telecommunication.
2. Description of the Related Arts
In general, in the receiver in a PCM telecommunication system, the received PCM signal is converted into parallel signals in a shift register, and the parallel converted signals from the shift register are supplied to a synchronization detection portion to produce a synchronization detection pulse indicating the existence of a synchronization pattern. The produced synchronization detection pulse is supplied to a synchronization protection portion having a main synchronization counter and a auxiliary (or sub-) synchronization counter to determine the synchronization state and carry out a predetermined protection.
In the operation of the synchronization protection portion, the number of sequences of preprotection (asynchronization confirmation) which give the criteria for the determination of asynchronization and the number of sequences of post-protection (synchronization recovery confirmation) which give the criteria for the determination of a synchronization recovery are selected with regard to the main synchronization counter and the auxiliary synchronization counter.
In the prior art, however, since the post-protection sequence number is usually selected to be the same for both the main synchronization counter and the auxiliary synchronization counter, sometimes an erroneous operation occurs in a pseudo synchronization detection portion in parallel with the synchronization detection portion if, for example, the error rate of the received PCM signal has deteriorated. Accordingly, a problem arises in that the timing from the auxiliary synchronization counter in a pseudo synchronization state is written into the main synchronization counter in the true synchronization state to bring the main synchronization counter to a pseudo synchronization state (simulated frame alignment state), and therefore, a stable frame synchronization cannot be established.